If this wasn't a recent occurrence, and there was any chance he was with other rabbits before you got him, that would be the most likely cause. Another rabbit bit him and ripped out a chunk of fur. If this is a possibility, then I would just make sure that it continues to heal well, and no lumps or redness occur. The fur will grow back in, usually with the next molt cycle.
If he hasn't been around other rabbits or animals, could he have had any injections at the vet prior to you getting him?
If it's recent and happened since he's been with you, cats can reach through bars. Could that be a possibility? Rabbits rarely will self mutilate, and if it's on his back or somewhere unlikely to be easily accessible by his own mouth, then self mutilation can be ruled out.
If you've ruled out other animals and him injuring himself on the cage, and self mutilation seems possible, then keep an eye on him and the area, and if it occurs again then he could be doing it because of a skin condition like mites, which would need to be treated with the correct rabbit safe med.
